Mg II absorbers: metallicity evolution and cloud morphology

TL;DR

This study investigates the metallicity evolution of Mg II quasar absorption systems using SDSS data, revealing their cloud morphology, density, and mass, and linking metallicity changes to star formation history.

Contribution

It provides new insights into the metallicity evolution, cloud density, and size of Mg II absorbers, and connects these properties to galaxy star formation processes.

Findings

Metallicity increases from z=2 to 0.5, reaching near-solar levels.

Median gas density of Mg II clouds is about 0.3 cm^{-3}.

Mg II clouds are numerous, small, and 'foamy', with masses around 10^3 solar masses.

Abstract

Metal abundance and its evolution are studied for Mg II quasar absorption line systems from their weak, unsaturated spectral lines using stacked spectra from the archived data of Sloan Digital Sky Survey. They show an abundance pattern that resembles that of Galactic halo or Small Magellanic Cloud, with metallicity [Z/H] showing an evolution from redshift z=2 to 0.5: metallicity becomes approximately solar or even larger at z~0. We show that the evolution of the metal abundance traces the cumulative amount of the hydrogen fuel consumed in star formation in galaxies. With the aid of a spectroscopic simulation code, we infer the median gas density of the cloud to be roughly 0.3 $\rm cm^{-3}$, with which the elemental abundance in various ionization stages, in particular C I, is consistently explained. This gas density implies that the size of the Mg II clouds is of the order of 0.03 kpc, which suggests that individual Mg II clouds around a galaxy are of a baryonic mass typically $\rm 10^{3} \, M_{\odot}$. This means that Mg II clouds are numerous and `foamy', rather than the large entity that covers a sizable fraction of galaxies with a single cloud.